Highly integrated electronic devices typically require consistent and dependable power supplies to provide reliable operation. For example, portable electronic devices, such as cell phones, personal digital assistance (PDA), electronic games and laptop computers, may include rechargeable batteries, which provide voltage over a wide range, depending on the state of the charge. For instance, when a 3.3V cell phone battery is nearly discharged, it may drop down to as low as 2.7V. However, once the battery is connected to a charger and fully charged, the voltage may be as high as 5.1V, for example, at least during the first period of device operation. Therefore, such electronic devices, which rely on rechargeable batteries (or other varying power source) must be capable of operating over a wide voltage range, e.g., approaching a 2:1 ratio.
Since battery life in portable electronic devices is always a concern, circuits within the portable electronic devices are generally designed to operate with the least possible current draw, while maintaining required gain and linearity. These design constraints are met at the lowest expected battery voltage (e.g., 2.7V in a cell phone). At higher voltages, the circuits usually pull additional current, which accelerates battery discharge and can even compromise performance.
FIG. 1 is a block diagram depicting a conventional bias circuit 100 of an electronic device, used to bias various transistor amplifier stages. The bias circuit 100 includes a power supply voltage source, e.g., a rechargeable battery, for providing a power supply voltage Vdd, connected to a current mirror 110 through resistor R101. The current mirror 110 includes bias or first transistor 111 and amplifier or second transistor 112. Although the second transistor 112 is shown simply as a single transistor for convenience of explanation, it is understood that the second transistor 112 is intended to be representative of various transistors, amplifier stages or other component(s) which are to be biased by the first transistor 111. AC components of the bias circuit 100 are not shown for clarity.
Each of the first and second transistors 111 and 112 may be field-effect transistors (FETs), for example, such as may be field-effect transistors (FETs) or gallium arsenide FETs (GaAsFETs). The first transistor 111 includes a source connected to a low voltage source (e.g., ground), a drain connected to the resistor R101 and a gate connected to a gate of the second transistor 112, as well as to the resistor R101. The second transistor 112 includes a source connected to the low voltage source (e.g., ground), a drain connected to the power supply voltage Vdd and a gate connected to the gate of the first transistor 111.
Referring to FIG. 1, there are two sources of power supply dependence on the drain current Id2 in the second transistor 112, generally speaking. The first source is the effect of the power supply voltage Vdd on the drain current Id1 of the first transistor 111, which is mirrored into the drain of the second transistor 112. The second source is the direct effect of drain-source voltage Vds on the drain current Id2 within the amplifier transistor 112 itself. The drain-source voltage Vds is the voltage across the drain and source of the second transistor 112, which is equivalent to the voltage at node Vd2 when the source of the second transistor 112 is connected to ground. The second effect is depicted in FIG. 2, for example, which includes graph 200 showing current versus voltage (I-V) characteristics with respect to Id2 and Vds in the second transistor 112.
As shown in FIG. 2, a significant amount of voltage dependent current in the second transistor 112 is seen from the characteristics of the second transistor 112 alone. For example, the graph of FIG. 2 depicts an attempt to maintain 10 mA of drain current Id2 in the second transistor 112 by fixing gate-source voltage Vgs. The gate-source voltage Vgs is the voltage across the gate and source of the second transistor 112, which is equivalent to the voltage at node Vg2 when the source of the second transistor 112 is connected to ground. However, variation in the drain-source voltage Vds of the second transistor 112 is responsive to variations in power supply voltage Vdd. This causes, for example, an undesirable dependence of the drain current Id2, in which the drain current Id2 continues to increase (i.e., above the target current of 10 ma) as the drain-source voltage Vds increases in response to the power supply voltage Vdd increasing. This direct correspondence may result in excessive current when the battery is in a fully charged state, as well in starvation as the battery discharges.